1. Field of the Invention
The present invention relates to an internal combustion engine fuel injection control method of raising mainly low-temperature startability, gasoline mileage, and exhaust-gas cleanness of an internal combustion engine where fuel is injected into an intake air flow path.
2. Description of the Related Art
In an internal combustion engine utilizing a method of injecting fuel into an intake air flow path, deterioration in low-temperature startability, gasoline mileage, exhaust-gas cleanness, which is caused through adhesion of the injected fuel onto the wall face of the intake air flow path, is of a particular problem.
As prevention measures for fuel adhesion to the wall face of an intake air flow path, fuel injection directed to an intake valve is effective. However, in the case where that kind of fuel injection is performed in synchronization with intake-air inflow into a cylinder, unevaporated fuel particles directly flow into the cylinder; therefore, no fuel-air mixture suitable for combustion can be obtained by the time combustion starts, whereby deterioration in startability, gasoline mileage, and exhaust-gas cleanness is caused eventually.
In the case where that kind of fuel injection is performed before the intake valve is opened, almost all of the injected fuel once adheres to the surface of the intake valve. When the internal combustion engine is still cool, the intake valve opens with most of the fuel being adhered to the surface of the intake valve; thus, the fuel, as a mass of liquid, flows into the cylinder or it is returned into the intake air flow path due to the effect of in-cylinder gas that is blown back to the inside of the intake air flow path and then, as a relatively large droplet fuel, flows into the cylinder along with the intake air. As a result, no fuel-air mixture suitable for combustion can be obtained by the time the combustion starts, whereby deterioration in startability, gasoline mileage, and exhaust-gas cleanness is caused eventually.
Accordingly, to date, in Japanese Patent Application Laid-Open No. 2003-239796, for example, there has been disclosed a technology in which the intake valve is opened during the exhaust stroke and fuel injection is implemented in synchronization with the timing when the valve is opened, the in-cylinder gas that is blown back into the intake air flow path due to the opening of the intake valve is made to collide with the injected fuel so that while fuel adhesion to the wall face of the intake air flow path or the surface of the intake valve are prevented, the evaporation of the fuel and the mixture of the fuel and air are facilitated.
In the technology disclosed in Japanese Patent Application Laid-Open No. 2003-239796, in order to make all the amount of injected fuel collide with in-cylinder gas that reversely flows, more reverse flow is required. However, in the case of normal valve overlap setting, the intake valve starts to open immediately before the piston reaches the top dead center of the exhaust stroke; therefore, there can be obtained only a little amount of reverse flow. Accordingly, in the technology disclosed in Japanese Patent Application Laid-Open No. 2003-239796, the intake valve is temporarily opened at a time when the moving speed of the piston is high and the exhaustion of in-cylinder gas is active, so that reverse flow of in-cylinder gas to the intake air flow path, which is extremely strong compared with the normal condition, is obtained.
However, in the case where fuel is injected toward the extremely strong reverse flow produced in such a way as described above, part of the injected fuel is strongly pushed back by the reverse flow and adheres, for example, to the upstream wall face of the intake air flow path. Because the inflow of the fuel that has adhered, for example, to the upstream wall face of the intake air flow path is delayed, the amount of fuel that is injected in a cycle does not coincide with the amount of fuel that substantially flows into the cylinder; therefore, there still remains the problem of deterioration in startability, gasoline mileage, and exhaust-gas cleanness.
Moreover, the rate of inert gas included in the in-cylinder gas that is blown back differs depending on the result of previous-cycle combustion; thus, when a greater amount of reverse flow is produced, especially, at the starting timing when the combustion condition is unstable, the fluctuation of substantial air-fuel ratio of a fuel-air mixture produced inside the cylinder becomes large, whereby deterioration in startability and exhaust-gas cleanness becomes more conspicuous.